f2fs: fix some ambiguous comments

After commit d7e9a903

 ("f2fs: Support Block Size == Page Size"),
Some comments are confused and just correct with block size is 4KB.

Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>